The Characterization of Extreme Episodes of Wet and Dry Deposition of Pollutants on an Above Cloud-Base Forest during its Growing Season

Abstract

An analysis of a 3-yr database (1986?88) acquired new Mount Mitchell (35°44?05?N, 82°17?15?W, 2038 m MSL) where the forest consists primarily of Fraser fir and some red spruce stands is presented. The site was immersed in clouds for 28%?41% of the time during each of the three growing seasons (15 May?15 September). This study only investigated extreme episodes of wet (cloud-water pH% ≤.3:1)and dry (eg., an ozone concentration ≥ 70 ppb) acidic deposition. Extreme wet events occasionally relieved periods of high ozone (≥ 70 ppb) exposures during the final field intensive. Extreme wet and dry events could activate the decline mechanism in any above cloud-base forest, especially if the trees are exposed to such events during very early or very late stages of their Lives. The exposure of the forest to natural climatic stress, such as drought condition wintertime temperatures during the growing season, snow storm during early spring, etc., would also subject the forest to a stressful period during which the exposure to the aforementioned episodes of pollutant deposition might trigger a decline. On the average, one of three cloud events that traverse this site is extreme. These extreme events usually last about 4 h. form during periods of high atmospheric pressure, have a liquid water content of 0.10 g m?3, and contain cloud droplets of mean diameter around 8.0 ?m. During the dissipating stages, such cloud events result in maximum acidic deposition. When such events are preceded by very high ozone (≥ 100 ppb), they may prove oven more detrimental to forest health. A precipitating cloud event (pH = 4.4 on the average) preceded by periods of very high ozone concentrations will become an extreme episode. Extreme acidic events can occur in association with 1) an 850-mb closed low, situated just north of Montreal, Canada, that advances southward into New York State, and 2) an 850-mb high extending over the Gulf of Mexico (between Florida and Louisiana) to over eastern Kansas. In contrast, an 850-mb low over the Indiana-Illinois-Kentucky border seems to be associated with nonextreme cloud events as it sets up a southwesterly-southeasterly flow into the site. Similarly, high pressure systems located north of the site (which travel from northern Ohio into Indiana and southward), and low pressure systems (which move between southern Quebec north of Lake Ontario and the New York State?Canadian border) are conducive to measurements of ozone exceeding 100 ppb at our site. Extreme cloud events may be characterized into four unique temporal pH patterns, each with a subpattern that includes rain. These patterns are found to be related to the physical characteristics of the sampled clouds. and to the synoptic and local meteorological conditions associated with them. A relationship between liquid water content and pH appears to exist for these events and seems to be dependent on the dynamics of their formation.